杏吧原创

THE romantic notion that early humans lived in harmony with their environment has taken quite a battering lately. Modern humans may have started eliminating other species right from the start: our ancestors stand accused of wiping out megafauna 鈥� from giant flightless birds in Australia to mammoths in Asia and the ground sloth of North America 鈥� as they spread across the planet.

Even so, by around 6000 years ago there were only about 12 million people on Earth 鈥� less than a quarter of the current population of Great Britain. That鈥檚 a far cry from today鈥檚 6.6 billion, many of us guzzling fossil fuels, churning out greenhouse gases and messing with our planet鈥檚 climate like there鈥檚 no tomorrow. So it may seem far-fetched to suggest that humans have been causing global warming ever since our ancestors started burning and cutting forests to make way for fields at least 7000 years ago.

Yet that鈥檚 the view of retired climate scientist William Ruddiman, formerly of the University of Virginia, Charlottesville. Ancient farmers were pumping climate-warming carbon dioxide and methane into the atmosphere long before recorded history began, he says. Far from causing catastrophe, however, early farmers halted the planet鈥檚 descent into another ice age and kept Earth warm and stable for thousands of years.

Hugely controversial

Could a few primitive farmers really have changed the climate of the entire globe? If you find this hard to believe, you鈥檙e not the only one. Ruddiman鈥檚 idea has been hugely controversial ever since he proposed it in 2003. 鈥淢ost new ideas, especially controversial ones, die out pretty fast. It doesn鈥檛 take science long to weed them out,鈥� he says. Yet five years on, his idea is still not dead. On the contrary, he says the latest evidence strengthens his case. 鈥淚t has become clear that natural explanations for the rise in greenhouse gases over the past few thousand years are the ones that are not measuring up, and we can reject them,鈥� he claims.

There is no doubt that the soaring levels of carbon dioxide and other greenhouse gases we see in the atmosphere today 鈥� causing a 0.7 掳C rise in average global temperature during the 20th century 鈥� are the result of human activities. In the late 1990s, however, Ruddiman started to suspect that our contribution to the global greenhouse began to become significant long before the industrial age began. This was when an ice core drilled at the Vostok station in Antarctica revealed how atmospheric CO₂ and methane levels have changed over the past 400,000 years. Bubbles trapped in the ice provide a record of the ancient atmosphere during the past three interglacials.

What we see is a regular pattern of rises and falls with a period of about 100,000 years, coinciding with the coming and going of ice ages. There is a good explanation for these cycles: periodic changes in the planet鈥檚 orbit and axis of rotation alter the amount of sunlight reaching the Earth (see 鈥淭he ice ages鈥�). We are now in one of the relatively brief, warm interglacial periods that follow an ice age.

Regular patterns

Within this larger pattern there are regular peaks in methane every 22,000 years that coincide with the times when the Earth鈥檚 orbit makes summers in the northern hemisphere warmest. This makes sense, because warm northern summers drive strong in southern Asia that both encourage the growth of vegetation and cause flooding during which vegetation rotting in oxygen-poor water will emit methane. Around the Arctic, hot summers thaw wetlands for longer, again promoting both vegetation growth and methane emission.

In recent times, however, this regular pattern has changed. The last methane peak occurred around 11,000 years ago, at about 700 parts per billion, after which levels began to fall. But instead of continuing to fall to what Ruddiman says should have been a minimum of about 450 ppb today, the atmospheric methane began to climb again 5000 years ago. 鈥淚t just went the wrong way,鈥� Ruddiman says.

Intrigued, he turned to records for CO₂. No one understands exactly why the level of atmospheric CO₂ varies naturally 鈥� a complex combination of factors is involved, from the growth of vegetation to volcanic activity. The Vostok core, however, shows that CO₂ levels correlate closely with temperature, peaking at 280 to 300 parts per million as an ice age ends, then falling to lower values. (Today the level is 384 ppm and rising ever faster.)

This pattern was repeated until about 7000 years ago, when CO₂ bucked a downward trend and started to rise again. Just before the industrial era began it was already around 40 ppm higher than would be expected based on the last three previous interglacial periods.

Canadian ice sheet

The effect of the rising levels of these greenhouse gases on Earth鈥檚 climate would have been significant. The extra 250 ppb of methane and the extra 40 ppm of CO₂ would have kept the average global temperature nearly 0.7 掳C warmer than it would otherwise have been, equalling the warming during the 20th century due to industrial emissions.

Working with climate modellers Stephen Vavrus and John Kutzbach of the University of Wisconsin-Madison, Ruddiman has shown that if the levels of these gases had continued to fall rather than rising when they did, ice sheets would now cover swathes of northern Canada and Siberia. The world would be heading into another ice age.

So why did both methane and CO₂ rise over the past few thousand years? In other words, why has this interglacial been different from previous ones. Could humans be to blame?

Forest clearing

Agriculture emerged around the eastern Mediterranean some 11,000 years ago, then shortly afterwards in China, and several thousand years later in the Americas. Farming can release greenhouse gases in various ways: clearing forests liberates lots of stored carbon as the wood rots or is burned, for instance, while flooded rice paddies release methane just as wetlands do.

To find out more about early farming, Ruddiman began to dig around in studies of agricultural history. These revealed that there was a sharp rise in rice cultivation in Asia around 5000 years ago, with the practice spreading across China and south-east Asia. Here at least was a possible source for the unexpected methane rise.

In Europe, people had begun clearing forests to grow cereal crops such as barley and wheat 7500 years ago (see graphic). There is no firm figure for the total extent of this forest clearance, but Ruddiman says it could have been vast. One pointer to this is the , which documents the 11th-century census of England ordered by William the Conqueror. 鈥淭here were 1.5 million people and they had cleared 85 per cent of the forest,鈥� Ruddiman says.

When he published his theory in 2003 (), there was no shortage of criticism. The most devastating came from Fortunat Joos at the University of Bern, Switzerland, who pointed out that there was simply not enough forest on Earth to account for the anomalous rise in CO₂. Cutting down every tree on the planet wouldn鈥檛 explain the CO₂ rise seen in ice cores over the past 7000 years.

Amplified

Ruddiman has conceded that Joos was right, and now argues that deforestation and rice growing released enough CO₂ and methane to cause a little warming, which then triggered feedback mechanisms that released more CO₂, causing further warming and so on, amplifying the influence of early farmers.

That such feedback mechanisms exist is not in doubt. Climate scientists all agree that the magnitude of the temperature changes as the Earth has gone in and out of ice ages cannot be explained by orbital changes alone. The issue is whether early farmers could have released enough CO₂ and methane to trigger feedback effects large enough to explain the total observed rise in CO₂.

With the help of Vavrus and Kutzbach, Ruddiman is using climate models to explore whether this is plausible. The models suggest that the early human influence might have been great enough to keep the oceans warm compared with previous interglacials. This could have boosted CO₂ levels in two ways. 鈥淐arbon dioxide is less soluble if the oceans are warmer,鈥� says Ruddiman. 鈥淚t鈥檚 a bit like a soda drink bubbling off its gases on a warm summer鈥檚 day.鈥�

Warmer oceans

Warmer southern oceans would also mean less sea ice around Antarctica, increasing CO₂ levels by boosting gas exchange between the atmosphere and seawater. 鈥淏oth the observational data and our model suggest these two factors are players,鈥� Ruddiman says. 鈥淲hether they fill the entire gap in the CO₂ budget we can鈥檛 say, but they look very promising.鈥�

The feedback idea also overcomes another objection to the 鈥渆arly anthropogenic hypothesis鈥�. During photosynthesis, plants take up a slightly larger proportion of the carbon-12 in the atmosphere than of the heavier isotope, carbon-13. This means that if all the CO₂ came from deforestation 鈥� which reverses the process 鈥� there should have been a gradual rise in the proportion of carbon-12 in the atmosphere relative to carbon-13. Yet the ice cores show no such thing, a fact pointed out in by renowned climate scientist Wally Broecker of Columbia University鈥檚 Lamont-Doherty Earth Observatory in Palisades, New York.

If a feedback mechanism was at work, however, 75 per cent of the extra CO₂ would have come from the oceans, and the relatively small rise in the proportion of carbon-12 would be consistent with the only isotope record published so far, Ruddiman says.

What鈥檚 more, since Ruddiman first suggested his hypothesis, ice core records of atmospheric methane and CO₂ levels have been extended back to 800,000 years ago, covering another five interglacial periods, three similar to the present one. So we now have data for six comparable interglacials, all of which suggest the rise in the gases over the past few thousand years is unprecedented, he claims. 鈥淣atural explanations for the rise have been tested six times and they failed six times 鈥� 12 times if you allow for the two gases.鈥�

Insane argument

Compelling? Emphatically not, says Broecker, who is Ruddiman鈥檚 most fervent critic. 鈥淚t鈥檚 an insane argument,鈥� he says. 鈥淚t鈥檚 total and utter nonsense.鈥�

鈥淚f Ruddiman wanted to argue that over the last thousand years there had been an effect, that would be harder to argue against because maybe there was,鈥� adds Broecker. 鈥淏ut if you look at that record during the last thousand years, CO₂ sort of flattens out. Just at the time you鈥檇 think that man鈥檚 effect would be the strongest, you see the least effect.鈥�

Ruddiman argues that this is because deforestation had levelled out. 鈥淭here鈥檚 good evidence that most of Eurasia was deforested by the time of Christ,鈥� he says.

Broecker reckons there鈥檚 a natural explanation for the CO₂ rise. The deep oceans store lots of carbon as solid calcium carbonate, and deep-sea sediments record a drop in carbonate concentrations that could account for the rise in atmospheric CO₂.

Previous interglacials

But if the CO₂ rise before the industrial age is natural, why did it not happen in previous interglacial periods? It did, Broeker claims, during the interglacial period 400,000 years ago (see graphic). He compares this interglacial with the present one by aligning them from the point at which the previous ice age ended, which shows CO₂ naturally staying high, above 270 ppm, for about 28,000 years. Ruddiman aligns the interglacials on the basis of the point in the orbital cycle at which incoming solar radiation is at a minimum, signalling the end of the interglacial. This paints a very different picture.

Other climate experts say it鈥檚 not possible to match the two cycles exactly because the three orbital parameters (see graphic) were different, so any choice is somewhat arbitrary.

Another of Broecker鈥檚 complaints is that Ruddiman has handed global-warming deniers some cheap ammunition: if ancient farmers headed off a looming ice age, isn鈥檛 that a good thing? 鈥淚t confuses the situation and we don鈥檛 need that,鈥� says Broecker. 鈥淧eople use it in some curious ways to speak against the effects of global warming,鈥� he adds. 鈥淚 get very emotional about it, yes, because I think it鈥檚 very bad science.鈥�

A lot of progress

Some climate-change deniers did pounce on Ruddiman鈥檚 hypothesis when it was first publicised, but largely dropped it when it was pointed out that they were effectively conceding that humans really do cause climate change. Many researchers, by contrast, remain intrigued by Ruddiman鈥檚 idea. 鈥淲e know that humans have had a lot of influence,鈥� says Richard Alley, a geologist at Penn State University in University Park, Pennsylvania. 鈥淏ill鈥檚 idea is that you see it in the atmosphere. He may be right.鈥�

鈥淏ill鈥檚 idea is that early human influence can be seen in the atmosphere. He may be right鈥�

Alley is impressed by a paper that Ruddiman and his colleagues published this year in which shows that the number of new rice-growing sites appearing in China from 6000 to 4000 years ago was 10 times that in previous millennia. 鈥淚 would not say Ruddiman wins, it鈥檚 over, let鈥檚 go home 鈥� but I think that he鈥檚 making a lot of progress with the methane.鈥�

Geochemist Ed Brook at Oregon State University in Corvallis isn鈥檛 so sure. He says the methane rise over the past 5000 years could have arisen from wetlands in the southern hemisphere. 鈥淭here are some nice records from caves in South America that show that region was getting wetter.鈥�

Circumstantial

On CO₂, things are even more uncertain. 鈥淚t鈥檚 a tough one, because the evidence is primarily circumstantial,鈥� Brook says. 鈥淚鈥檓 just as willing to think that there are some natural mechanisms that we don鈥檛 understand.鈥� Subtle changes in the Earth鈥檚 orbit might drive feedback that makes the atmospheric chemistry very different from one interglacial period to the next.

Despite the doubts, Brook welcomes Ruddiman鈥檚 research. 鈥淗e has questioned the conventional wisdom in a way that I think is productive,鈥� he says. 鈥淚t makes us really stop and think.鈥�

Future results should help resolve matters. Measurements of carbon and hydrogen isotope ratios in methane from ice cores could provide subtle clues to the methane鈥檚 origin, while comparisons of methane levels in cores from Antarctica and Greenland might reveal where the extra methane came from geographically. The existing measurements are not conclusive but suggest the extra methane came from low latitudes, which is consistent both with Ruddiman鈥檚 hypothesis and with natural methane from tropical South America.

Ruddiman, meanwhile, plans to continue working on climate models and researching the history of human land use. 鈥淚f we don鈥檛 understand something as basic as why greenhouse gases have increased in the past several thousand years, then there鈥檚 a huge gap in our knowledge of the climate,鈥� he says.

For now, it is still an open question: did ancient farmers kick off global warming or not? The answer is in a sense academic, because it has no bearing on the far more rapid warming caused by industrialised societies. If it鈥檚 true, though, it could be seen as a warning. If a few million people wielding stone axes averted an ice age, just imagine what the legacy of their 7 billion oil-guzzling descendants will be.

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